In digital transmission systems, a dual or redundant system structure is commonly employed for increased system reliability. A system is duplicated to provide two sets of outputs. The outputs may be compared to detect errors. When a bad output is detected, the subsequent components of the system may be switched to use the good output.
There is a need for a simple and efficient circuit for checking the outputs of a dual system to detect errors occurring on either of the two system outputs or buses. Furthermore, there is a need for a circuit of this character which also produces an error indication in response to an internal failure within the error checking circuit itself, to afford a self-checking circuit. There is a further need for a circuit of this character which actively exercises the detection circuitry in the absence of an error to provide an early warning of a fault within the detection circuit and hence prevent silent failures. There is a still further need for a circuit which can additionally identify which of the buses has failed, for the most common types of failures.